Touchscreen displays are able to detect a touch such as by a finger or stylus within an active or display area. Use of a touchscreen as part of a display enables a user to interact with an electronic application by touching the touchscreen. The display may present images to the user. Such images may include user interface constructs such as different buttons, images, or other regions that can be selected, manipulated, or actuated by touch. Touchscreens can therefore provide an effective user interface for cell phones, GPS devices, personal digital assistants (PDAs), computers, ATM machines, appliances, and other devices.
Touchscreens use various technologies to sense touch from a finger or stylus, such as resistive, capacitive, infrared, and acoustic sensors. In capacitive sensor based touchscreens, a touch changes a capacitance at a node in an array of electrodes overlaying the display device. Capacitive touchscreens often use two separate layers of transverse electrodes arranged as an X-Y matrix separated by a dielectric layer. The regions proximate the intersections of the transverse electrodes form sensing nodes, which are individually accessed by a sequential scanning process to determine the location of one or more touches. Transparent electrodes made from indium tin oxide (ITO) or transparent conductive polymers, or fine metal lines may be used to form the array of nodes over a liquid crystal display (LCD). Images on the LCD display can be seen through the transparent capacitive touchscreens.
LCD displays may emit alternating electric fields that can interfere with touch detection. In some prior touchscreen devices, an additional solid ITO layer was used as a shield between the electrodes layers and the LCD. This added significant expense in materials as well as processing. In other prior touchscreen devices, a layer of electrodes closest to the LCD, referred to as drive electrodes, was made up of wide electrodes that substantially covered the LCD, providing a shield for a top layer of electrodes referred to as sense electrodes. Some prior devices utilized intermediate drive electrodes 150, 155 as shown in FIG. 1, along with a network of resistors 160 to couple the intermediate drive electrodes from the primary X drive signals as shown. The use of intermediate drive electrodes allowed a reduction in the number of X drive lines directly driven by the control circuitry, reducing the complexity and pin count of the control circuitry and the number of required connection wires. However, the resistors required space, increased cost, and also degraded manufacturing yield. It also created additional loading on the control circuitry, dissipated more power, and degraded the shielding capability of the electrodes against LCD noise due to raised effective impedance levels of the electrodes caused by the introduction of the divider resistors.